lang/C/the.ansi.c.programming.language/c.programming.notes.int/sx2h.html

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   9 <title>16.8: <TT>EOF</TT> and Errors</title>
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  15 <H2>16.8: <TT>EOF</TT> and Errors</H2>
  16
  17 <p>When a function returns <TT>EOF</TT>
  18 (or, occasionally, 0 or <TT>NULL</TT>,
  19 as in the case of <TT>fread</TT> and <TT>fgets</TT> respectively),
  20 we commonly say that we have reached ``end of file,''
  21 but it turns out that it's also possible
  22 that there's been some kind of I/O error.
  23 When you want to distinguish between end-of-file and error,
  24 you can do so with the <TT>feof</TT> and <TT>ferror</TT> functions.
  25 <TT>feof(fp)</TT> returns nonzero
  26 (that is, ``true'')
  27 if end-of-file has been reached on the stream <TT>fp</TT>,
  28 and <TT>ferror(fp)</TT> returns nonzero
  29 if there has been an error.
  30 Notice the past tense and passive voice:
  31 <TT>feof</TT> returns nonzero if end-of-file
  32 <em>has been reached</em>.
  33 It does <em>not</em> tell you that the next attempt to read from
  34 the stream will reach end-of-file,
  35 but rather that the previous attempt
  36 (by some other function)
  37 already <em>did</em>.
  38 (If you know Pascal,
  39 you may notice that the end-of-file detection situation in C
  40 is therefore
  41 quite different from Pascal.)
  42 Therefore, you would never write a loop like
  43 <pre>
  44         while(!feof(fp))
  45                 fgets(line, max, fp);
  46 </pre>
  47 Instead, check the return value of the input function directly:
  48 <pre>
  49         while(fgets(line, max, fp) != NULL)
  50 </pre>
  51 With a very few possible exceptions, you don't use <TT>feof</TT>
  52 to <em>detect</em> end-of-file;
  53 you use <TT>feof</TT> or <TT>ferror</TT> to distinguish
  54 between end-of-file and error.
  55 (You can also use <TT>ferror</TT> to diagnose error conditions on
  56 output files.)
  57 </p><p>Since the end-of-file and error conditions tend to persist on a stream,
  58 it's sometimes necessary to clear (reset) them,
  59 which you can do with <TT>clearerr(FILE *fp)</TT>.
  60 </p><p>What should your program do if it detects an I/O error?
  61 Certainly, it cannot continue as usual;
  62 usually, it will print an error message.
  63 The simplest error messages are of the form
  64 <pre>
  65         fp = fopen(filename, "r");
  66         if(fp == NULL)
  67                 {
  68                 fprintf(stderr, "can't open file\n");
  69                 return;
  70                 }
  71 </pre>
  72 or
  73 <pre>
  74         while(fgets(line, max, fp) != NULL)
  75                 {
  76                 <I>... process input ...</I>
  77                 }
  78
  79         if(ferror(fp))
  80                 fprintf(stderr, "error reading input\n");
  81 </pre>
  82 or
  83 <pre>
  84         fprintf(ofp, "%d %d %d\n", a, b, c);
  85         if(ferror(ofp))
  86                 fprintf(stderr, "output write error\n");
  87 </pre>
  88 Error messages are much more useful,
  89 however,
  90 if they include a bit more information,
  91 such as the name of the file for which the operation is failing,
  92 and if possible <em>why</em> it is failing.
  93 For example,
  94 here is a more polite way to report that a file could not be opened:
  95 <pre>
  96         #include &lt;stdio.h&gt;        /* for fopen */
  97         #include &lt;errno.h&gt;        /* for errno */
  98         #include &lt;string.h&gt;       /* for strerror */
  99
 100         fp = fopen(filename, "r");
 101         if(fp == NULL)
 102                 {
 103                 fprintf(stderr, "can't open %s for reading: %s\n",
 104                                         filename, strerror(errno));
 105                 return;
 106                 }
 107 </pre>
 108 <TT>errno</TT> is a global variable, declared in <TT>&lt;errno.h&gt;</TT>,
 109 which may
 110
 111 contain
 112 a numeric code
 113 indicating the reason for a recent system-related error
 114 such as inability to open a file.
 115 The <TT>strerror</TT> function takes an <TT>errno</TT> code
 116 and returns a human-readable string
 117 such as ``No such file'' or ``Permission denied''.
 118 </p><p>An even more useful error message,
 119 especially for a ``toolkit'' program
 120 intended to be used in conjunction with other programs,
 121 would include in the message text
 122 the name of the program reporting the error.
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